This invention relates to an automated gray level transformation system for use in an image processing work station where a plurality of digital images are displayed for the purpose of comparative observation, and more particularly to an automated gray level transformation system in which the contrast of an objective image can be automatically changed.
As an example of a prior art method for automated gray level transformation, a method for histogram flattening or equalization transformation is described in a book entitled "Digital Picture Processing" pp. 172-177, written by A. Rosenfeld and A. C. Kak and published in 1976 by Academic Press.
According to this histogram equalization transformation method, a picture's histogram is transformed so that all of the intensity values appear with the same frequency. Thus, the amount of information in the picture can be maximized.
When an image is processed according to this histogram equalization transformation method, the contrast in a region including intensity occurring with a high frequency is improved, while, on the other hand, the contrast in a region including intensity occurring with a low frequency is lowered.
The prior art method described above has been effective only when an objective image and an image display system satisfy the following conditions:
(1) A background region (a meaningless region other than the objective region) does not substantially exist in the image.
(2) The frequency of intensity in the objective region of the image does not have a small value over a wide range.
(3) The frequency of intensity in the objective region of the image does not have an extreme Peak.
(4) The image display system has a linear input/output characteristic throughout its operating range.
The prior art method has had the problem that the desired result of image processing cannot be achieved when the objective image and the image display system do not satisfy the conditions (1) to (4) described above. More precisely, in the case of imaging for the medical diagnostic purpose, such as X-rays imaging or MRI (magnetic resonance imaging), the conditions (1) to (3) are hardly satisfied, and, in the case of CRT or like image display, the condition (4) is difficult to be satisfied due to the unsatisfactory image displaying characteristic especially in a low intensity portion.
Therefore, when the prior art method is directly applied to the imaging for the medical diagnostic purpose, a contrast shortage appears in the objective region in the case of the condition (1), because the background region occupies a wide intensity range. Also, in the case of the condition (2), the objective region has a narrowed intensity range, and a contrast shortage occurs in the objective region.
Also, in the case of the condition (3), the intensity range in the objective region is unnecessarily greatly widened, and an artifact due to an overemphasis and generation of noise are given rise to.
Further, in the case of the condition (4), the intensity in low intensity portions tends to be depressed, and the observability for dark portions tends to be lowered in conjunction with the visibility of the human visual system.